Firstly
to cable forum
I think your Scart Adaptor 2 into 1 is probably one of the problems. You want to be able to switch manually or automatically between the different signals. However you now have three SCART based devices all trying to connect into one TV. That won't go. You need to get a new scart multiadaptor with at least three inputs to one output (get a four way one for future proofing for when one day someone want's a games console
Wiring now becomes simple with the adaptor.
Scart Adaptor output > TV Scart in
STB output (TV) > Scart Adaptor input #1
DVD output > Scart Adaptor input #2
VCR Scart AV1 > Scart Adaptor input #3
STB output (VCR) > VCR Scart AV2 (in/decoder)
Now comes the clever bit with the coaxial aerials. Get yourself one of those "y" splitter things available most DIY type stores.
STB coax out > Y splitter
rooftop aerial > Y splitter
Y splitter tail (single side) > TV
If you wish you can loop the aerial feed via the VCR, coaxial in and out but theres no point including the DVD in that as you would inevitably use the DVD's scart for playback for the best picture.
You need to ensure the STB, DVD and VCR do not transmit their signals on the coaxial over the same channel frequency, whether or not they are connected via coaxial. They are close enought toeach other to potentially cause interference.